Bisphenol A is an important commercial intermediate in the manufacture of epoxy, polycarbonate, phenyoxy, polysulfone, and certain polyester resins. It is also used in flame retardants and rubber chemicals. A derivation of bisphenol A is possible by a condensation reaction of phenol and acetone catalyzed by HCl at 150.degree. F.
The p,p'-isomer of bisphenol is commercially favored over the o,p'-isomer and for that reason separation of these two isomers is appropriate. Commercial market expectations of lower o,p'-isomer concentrations have required need of better separation procedures. Current bisphenol A purification processes include crystallization from a solution of wet bisphenol in toluene as disclosed in U.S. Pat. No. 4,354,046; adductive crystallization with phenol as shown in U.S. Pat. Nos. 4,156,098 and 2,791,616; solvent leaching of solid bisphenol as in U.S. Pat. Nos. 3,493,622 and 3,207,795; recrystallization by cooling of a water-wet melt disclosed in U.S. Pat. No. 3,326,986; recrystallization by cooling of a solution in various organic solvents as in U.S. Pat. No. 4,242,527; and recrystallization by adding cosolvents to a solution of bisphenol in an organic solvent as disclosed in U.S. Pat. Nos. 3,919,330 and 4,192,955.
This invention discloses a novel and unobvious separation technique which utilizes a different principle from the above procedures: crystallization of bisphenol A isomers through removal of a cosolvent by azeotropic distillation. The solubility of p,p'-bisphenol in toluene at temperature as high as 98.degree. C. is only 8.6% by weight. With water present at an amount equal to 10% of the weight of bisphenol, formation of a stable liquid solution containing equal quantities of bisphenol and toluene to temperatures as low as 80.degree. C. is possible. This phenomenon is likely due to a synergistic solvent effect related to the depression of the bisphenol freezing point by a small amount of water: 10% water in o,p'-bisphenol will depress its freezing point from 108.degree. C. to 70.degree. C., and 17% water in p,p'-bisphenol will depress its freezing point from 157.degree. C. to 90.degree. C. This data on the solubility of bisphenol in water and toluene, separately and together, sparked curiosity concerning the behavior of a bisphenol/water/toluene solution as the water is removed by azeotropic distillation. This invention is the result of investigating that curiosity; a high yield one step separation technique by azeotropic distillation of the primary isomers of bisphenol A.